All cells contain an internal resistance caused by factors such as the
plate material and size etc. In primary cells this resistance is quite
high, with secondary cells due to the plates having a much larger
cross sectional area compared to primary cells the internal
resistance is considerably lower.
When cells and batteries are connected together the internal
resistance has to be taken into account so as to determine the
output characteristics, this is achieved in the following ways:
In order to obtain a high voltage from any cell type it is necessary to
connect the cells in series. In this arrangement the voltage from
each cell is added for example 6, x 2volt cells connected in series
will give an open circuit terminal voltage (EMF) of 12volts. The
disadvantage of a series arrangement will be that the internal
resistance of each cell is also added, this results in a high EMF
voltage but low current output, therefore an increase in current
drawn from the cell will result in a drop in the output voltage due to
the high internal resistance.
In this arrangement all the cells will be required to be of the same
hour rating, as this type of connection will take the lowest
ampere/hour rated cell as its output.
If cells are connected in parallel this will have the effect of the individual
cell voltage being the EMF voltage, due to the large surface area of the
all the cathode plates being connected together, and also all the anode
plates being connected together, the internal resistance is greatly
reduced, in this arrangement the current from each cell is added
together, therefore the current capacity from a parallel connected group
of cells is greatly improved, and as the EMF voltage is the same as the
cell voltage it is considerable more stable when a load is connected to
In this arrangement the cells ampere/hour rating does not need to be
considered, however the voltage rating of each cell needs to be equal
as this type of connection will reflect the lowest voltage of cell as its
From the 2 methods of connection above, it would make sense to
connect all cells in a battery in a series/parallel configuration, this
would provide the battery with a high voltage and also a high current
capacity with an overall lower internal resistance, in this
arrangement the power obtained from the battery would be at its
EMF (Terminal Voltage) and internal
Internal resistance in batteries is mainly due to the resistance of the
electrolyte and the cross sectional area of the plates. The voltage
that is measured across the terminals of the battery when it is off
load is known as the EMF voltage. When a load is connected to a
battery the current flows through the internal resistance and causes
a voltage drop proportional to this resistance, if the internal
resistance remains constant then the fall in the terminal voltage will
be proportional to the load current.
With switch “s” open, the Voltmeter V reads EMF (off load voltage).
With switch “s” closed, the Voltmeter reads Pd (the volt drop across
the load or on-load voltage).